You can set up a welcome page that shows up for every user who enters your classroom. This can be any static or dynamic web page that is publicly available in the internet.
To do so log on as administrator and visit Management -> Virtual Classrooms. Find your classroom in the list (if more than one classroom available in your configuration) and click on the red options button next to it, then click Settings.
Fill in the Welcome Page on Join (URL) field with your page address. Make sure to specify the fully qualified address including http:// or https:// in the beginning. Finally click Apply.
To remove the welcome page leave the filed empty.